This invention relates to a load controlled inverter in general and more particularly to an improved method and apparatus for operating such an inverter taking into consideration the protection time of the inverter thyristors and the commutating time of the inverter.
A method and apparatus of this nature is known in which, prior to each zero crossing of the load voltage at an instant which is determined from three measured voltages, where the first measured voltage is in phase with and proportional to the load voltage, the second measured voltage is phase shifted by about 90.degree. relative to the load voltage and is proportional to a variable derived from the load voltage, and the third measured voltage is proportional to the inverter output current are used to take thyristor protection time and inverter commutating time into consideration. The apparatus includes an inverter, the a-c output terminals of which are connected to a parallel resonant circuit as the load, and an evaluation circuit in which current sensors are arranged for measuring the inverter output current and voltage sensors for measuring the resonant circuit voltage. These are followed by a network which contains components shifting the phase by about 90.degree. and which is connected to the inputs of a limit indicator, the output of which is connected to the trigger input of a control unit which forms firing pulses for the thyristors of the inverter.
Such is described in German Offenlegungsschrift 2,008,239. In the disclosed apparatus the second measured voltage is proportional to the rate of change of the load voltage. An inverter with a parallel resonant circuit as the load, what is known as a parallel resonant circuit inverter, in which taps at the capacitor of the parallel resonant circuit are fed to a limit indicator via a differentiating network, which may, for instance, be a resistor in shunt with a capacitor or a resistor in series with a coil is disclosed. To separate the potentials, a transformer may be connected between the taps at the capacitor and the differentiating network. The limit indicator delivers a trigger pulse to the control unit and thereby releases firing pulses for the thyristors of the inverter when the current in the differentiating network has become zero. The position in time of the trigger pulses relative to the zero crossings of the load or resonant circuit voltage of the parallel resonant circuit is given by the resistance and capacitance or inductance values of the differentiating network, which are chosen so that each trigger pulse is ahead of the corresponding zero crossing of the resonant circuit voltage by a constant time period. The length of the time interval, which is constant for each half period of the resonant circuit voltage, is chosen greater than the protection or recovery time of the thyristors, whereby the commutating time of the inverter is taken into consideration. Since the commutating time is current dependent, the choice of a constant time interval is not satisfactory. In the known inverter, provision is therefore made to adapt the length of the time interval to different operating conditions by changing to different capacitors or coils. In addition, several evaluation circuits may be provided in the known inverter, in order to determine the commutating and protection time for greatly different operating conditions. Finally, an evaluating circuit in which the resonant circuit voltage and the differentiated resonant circuit voltage are determined via the described differentiating network and which has a current transformer for measuring the inverter output current, with a trigger pulse released if the sum of these three signals is zero is described for the known inverter. The impedances in the network, which determine the proportionality of the signals, are adjusted so that at lower operating frequencies, the resonant circuit voltage and the inverter output current, at higher operating frequencies the resonant circuit voltage and its differential, and at higher operating frequencies and large currents, all three signals determine the position of the trigger pulses and thereby, the firing instants. In the above-mentioned German Offenlegungsschrift it is explained that with this method, optimum adaption of the firing instant to the real operating conditions is not provided either.
Thus, there is a need for an improved method and apparatus of the type mentioned in which the commutating and protection times are detemined without delay and where these times correspond optimally to the real operating conditions.